This invention relates to the casting of components and in particular to such casting using cores to define passages in the components, e.g. cooling air passages in cast blades for use in gas turbine engines. The invention includes a method of casting components.
In casting such blades it is conventional to use cores of silica, this material being of moderate rigidity and refractoriness but easy leachability. Because of the limited rigidity and refractoriness of silica, it is often necessary, especially in the case of a long core or a core of complex shape, to support the core so as to prevent it from being deflected by the inflow of molten blade material in filling the mold or to prevent it from being distorted at high temperatures.
The problem of core distortion is particularly acute in the casting of directionally solidified and single crystal components wherein the mold and core are heated to a higher temperature (typically in excess of 1500.degree. C.) than in conventional casting, and are maintained at the higher temperature for a longer period of time.
It has been proposed in the past to make the core of tubular form and provide an inner re-inforcement.
For example in UK Pat. No. 1,514,819 a tubular core is lined by a re-inforcing material of greater strength bonded to the inner surface of the core. Such re-inforced cores have been found frequently to break during the casting process leading to an ill-defined passage in the cast component.
It is also known, for example, from UK Patent application No. 2,019,756 to put metal rods inside a ceramic sheath to act as a re-inforcement. The metal disclosed in this Patent application is copper. Clearly such a re-inforced core would be of no use in the casting of superalloy gas turbine engine blades in directionally solidified form, when the casting temperature of the mold is in excess of 1500.degree. C.
Another problem in the casting of turbine blades for gas turbine engines is that the cores are often required to be formed with bends therein, due to the misalignment between parts of the blade aerofoil which require cooling air passages therein, and the root of the blade through which the air is supplied to the aerofoil. This requirement calls for a core material which is deformable enough to be capable of being bent to the appropriate shape but which is rigid enough not to distort at the high temperatures required during its use. However, at temperatures in excess of 1500.degree. C. ceramic cores must be used, but these two requirements are incompatible with present ceramic core materials. This is because in order to provide the strength, the available core materials such as alumina, or silicon nitride are too rigid to be bent without great difficulty, and are considered to be non-leachable from the castings, or only leachable with great difficulty. On the other hand, the leachable and more easily deformable materials such as silica or the glass ceramics are unable to withstand the temperatures required, particularly for casting directionally solidified components, for the required length of time without deformation.